Thrust reversers are brake devices that have been in use for decades on turbojet powered airplanes during landings. Their function is, at least, to suppress residual thrust in jet engines towards the rear of the aircraft, even when these are slowed down during landing. Preferably, their function is to redirect a part of the flow of gas produced by the engine toward the front of the aircraft. This action brings about a deceleration of the airplane, facilitating the braking action.
The description that follows is limited to double-flow turbofan engines, such as those used as standard equipment for commercial aircraft. These turbofan engines comprise a ducted fan driven by the jet turbine, which generates what is known as a secondary airflow that circulates in an annular channel around the jet engine itself.
Nevertheless, flight reversers are devices used solely for assisting in the braking process, and not as the primary braking device. Furthermore, commercial aircraft certification is carried assuming that thrust reversers are not in use, as the braking system must be adequate to bring an airplane to a complete stop on its own. Nevertheless, airlines seek ways of improving safety and reducing operating costs for their fleets. Against this backdrop, thrust reversers are seen as a guarantee of safety, especially during landings in bad weather conditions, as well as during landings on wet airstrips or on locally iced runways.
In addition, from an economic perspective, the use of thrust reversers decreases the landing distance required for an airplane by an amount in the range of 25% to 50%, depending on whether the runway is dry or slippery, resulting in less wear on tires and brakes during operations. In parallel, their use naturally brings about a reduction of time aircraft are using the runway, which results in savings in fuel and airport taxes for companies and allows airports to bring in more traffic. It is understood that thrust reversers must produce the strongest possible thrust toward the front of the aircraft and thus divert the greater part of the flow of gas from the jet engine.
At present, three principal types of thrust reversers are known.
The most standard and widely used system at the current time is the cascade type of thrust reversers. With this type of thrust reverser, only the secondary flow is channeled, i.e. the air flow that circulates in the faired duct encompassing the motor. To accomplish this, an interior blocker door is used to block the secondary flow, with air exhausted outside by means of an opening in the external surface of the motor. This opening is uncovered when operating interior doors by moving backwardly, in the direction of gas flow, a sliding hatch which is located outside the nacelle. The opening comprises a series of grilles, or cascades, arranged so as to monitor the angle of deflection of the flow. This type of reverser has been described in the patent document EP1176302.
The second type of thrust reverser is the blocker doors reverser. This type of reverser is made up of doors incorporated into the nacelle, four doors for example, arrayed angularly in a consistent manner. These doors are hinged on their rearward parts around axes perpendicular to the gas flows at a tangent to the surface of the nacelle and are operated by system controlled actuators. In reverse thrust mode, the four doors are opened, with the lower part of the doors thus blocking secondary flow, while the upper part of the doors serves to redirect reflected air. This blocker door type of a reverse thrust device has been described in the patent document EP0851111.
A third type of thrust reverser is known as the clamshell or target-type of thrust reverser. This is a thrust reverser disposed downstream of the nacelle that contains two clamshells. The salient characteristic of this thrust reverser is that in the thrust reverse mode, the device reverses not only the secondary flow, but the primary flow as well, because the two clamshells form an obstacle to the entire flow coming out of the jet engine and then redirect it forward by defining openings on the sides of the nacelle. This type of reverser has been described in the patent document EP0774578.
The installation of thrust reversers has naturally been planned for all new commercial aircraft in view of the economic and landing safety advantages the devices represent as described above.
Among these, certain airplane designs call for engine mounting configurations to the aft of the fuselage with the objective of reducing noise and drag, in which the propulsion system would be semi-recessed, in other words, the nacelle would be partially incorporated into the fuselage.
It is clear that the existing thrust reversers, of cascade type or blocker door type, are not liable to be installed on these kinds of aircraft.